prime material dragon

Prime material dragons effect. If i actvate mirror force does my opponent have too discard one card for each monster that would be destroyed? ...if he attacks directly do i gain lifepoints. if someone used prime material dragon and was able to summon 3 blue eyes white dragons in three turns using red-eyes darkness metal dragon is it a fair to assume he/she is cheating? .... How do i get rid of prime material dragon?!

If you activate Mirror Force your opponent discards 1 card. It doesn't matter how many monsters it will/can destroy.

Losing LP in battle is not an effect so you don't gain any LP.

Don't understand third one completely, but if he used Red Eyes Darkness Metal Dragon's effect 1 time per turn, 3 turns in a row to summon 1 Blue Eyes each time, I see nothing wrong.

You can attack it with a monster with more than 2400 attack or use a card that doesn't destroy, such as Dimensional Prison, Caius, Brain Control and then tribute/synchro.